Enhancement of hydrogen and methane production through anaerobic digestion using pre-treatments

Dark fermentation of organic wastes is considered as a promising process in terms of sustainable waste management and simultaneous biofuel production. Pre-treatment technologies are known as essentials of dark fermentation to overcome obstacles responsible for low H2 yield. The effect of aerobic pre-treatment of food wastes with different compositions (carbohydrate-rich, protein-rich and lipid-rich) prior to two-stage anaerobic digestion, on H2 and CH4 productions was investigated. The results showed that pre-aeration of food waste did not constitute an effective treatment for the purpose of improving H2 production potential during the first stage of the AD process. However, during the subsequent stage of AD, CH4 yield for protein-rich substrate, increased by 45.6%, thus revealing that carbon conversion to CH4 had an increase after pre-aeration. In case of inoculum pre-treatment, a novel method using waste frying oil (WFO) was introduced. H2 production from glucose was investigated for inoculum pre-treated with different concentrations of WFO. In the next step, a flux balance analysis model was developed to study the effect of inoculum pre-treatment on H2 producing and H2 consuming metabolic pathways. The results showed that H2 consumption by hydrogenotrophic methanogens that was accounted for about 56% of the loss in the H2 yield in untreated cultures, was negligible when the inoculum pre-treated with WFO. Moreover, optimization of H2 yield from food waste was performed in the next step of research, using a three-factor three-level Box-Behnken design method. Initial pH, pre-treatment duration and waste frying oil concentration were considered as the experimental factors. The results showed that combination of high WFO concentration, low initial pH and long pre-treatment could result in inhibition of methanogens. Furthermore, two-stage anaerobic digestion of food waste was performed using the inoculum pre-treated with WFO and total energy yield was compared with three common pre-treatments (heat shock, aeration and alkaline pre-treatment) and untreated cultures. The results showed that inoculum pre-treatment with WFO resulted in higher H2 and CH4 productions compared to alkaline, aeration and heat shock pre-treatment. Finally, microbial community of inoculum at different stages of dark fermentation (untreated, pre-treated with WFO, washed and fermented) was investigated to understand the effect of inoculum pre-treatment with WFO on H2 producing and H2 consuming microbial populations. The microbial diversity analysis showed that inoculum pre-treatment with WFO did not affect spore-forming H2 producing bacteria. However, it resulted in increased relative abundances of non-spore forming H2 producers which could be considered as an advantage in comparison with harsh pre-treatments such as heat shock.